Use new-style s3 urls in docker download script

Author: alexcrichton

src/ci/docker/run.sh

@@ -42,7 +42,7 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
         awk '{print $1}')
 
       s3url="s3://$SCCACHE_BUCKET/docker/$cksum"
-      url="https://s3-us-west-1.amazonaws.com/$SCCACHE_BUCKET/docker/$cksum"
+      url="https://$SCCACHE_BUCKET.s3.amazonaws.com/docker/$cksum"
       upload="aws s3 cp - $s3url"
 
       echo "Attempting to download $url"